1. Field of the Invention
This invention relates generally to transistors, and particularly to a Heterojunction Bipolar Transistor (HBT) having a novel layout and an emitter having more than one feed point.
2. Background Art
The global wireless market is rapidly expanding as people demand products that increase the ability to communicate freely without time and place restrictions. These products includes the very popular cellular telephones that employ either analog or digital technology. Cellular telephones utilize a transmitter to transmit data to a local base-station that in turn forwards the data to another base-station or the intended party. An important component in the transmitter and in other wireless communication systems is the power amplifier. The design of power amplifiers employs power transistors. A common power transistor is the Heterojunction Bipolar Transistor (HBT).
FIG. 1 illustrates a layout of a conventional HBT 1 having a first section 2 (also referred to as an "active" area) with a collector terminal 7, a second section 3 with a base terminal 5, and a third section 4 with an emitter terminal 6. The first section 2 includes three rectangular collector contacts 10 and two rectangular base pedestals 11. Each base pedestal 11 includes a rectangular base contact 12 and a rectangular emitter 13. Emitter contacts (not shown) are deposited over the emitters 13.
A base ballast resistor 8 having a layout with two rectangular areas is coupled between a base terminal 5 and base contact 12. An emitter ballast resistor 9 having a layout with two rectangular areas includes a first end that is coupled to emitter terminal 6 and a second end that is coupled to the emitter contact.
FIG. 2 illustrates a cross sectional view of transistor 1 of FIG. 1 through line 2--2. This sectional view further illustrates that there are two separate base pedestals 11, two separate base contacts 12, two separate emitters 13, and three separate collector contacts 10 for each transistor 1. A first metal layer 14 and a second metal layer 15 are deposited and selectively etched away so as to contact portions of transistor 1, such as collector contact 10, base contact 12, and emitter 13.
Unfortunately, these conventional HBTs suffer from the following disadvantages: 1) large area; 2) susceptibility to emitter failure; and 3) poor electrical performance.
LARGE AREA
Since the area occupied by power transistors in a power amplifier is a significant portion of the total die area for a power amplifier, reducing the area of the power transistors is desirable since this can substantially reduce the total die area occupied by the power amplifier. As is well-known by those skilled in the art, a reduction of the total die area for a circuit would reduce the costs to manufacture the circuit and would also increase integration (i.e., the number of circuit elements that can be integrated into a circuit design).
However, the ability to reduce the area of circuits is hindered by heat concerns as systems migrate to lower voltages. As the voltage of the system decreases, designers are forced to make the transistors larger to accommodate a fixed power requirement (i.e., P=VI, where P is the power, V is the voltage, and I is the current). If V decreases, such as in a low operating voltage environment, then I must increase to keep P constant. However, an increase in I is accomplished through a larger transistor area and in particular a larger emitter area. Therefore, conventional power transistors 1 that are designed for use in low voltage systems have layouts with large area requirements in order to handle the high current levels.
Consequently, it is a challenge to design transistors that can handle the current requirements of a low operating voltage system while simultaneously maintaining or shrinking the layout area of the transistor.
SUSCEPTIBILITY TO EMITTER FAILURE
Moreover, in conventional power transistors 1, the current density in the emitters 13 is uneven. In other words, there is a low current density at a first end 13A of emitter 13 while there is a high current density at a second end 13B of emitter 13. Since a high current density exposes the second end 13B of the emitter 13 to extremely high temperatures, the transistor 1 is susceptible to failure stemming from emitter 13 failure (e.g., emitter burn-out).
POOR ELECTRICAL PERFORMANCE
Conventional power transistors 1 also suffer from a high base-to-collector capacitance that impairs electrical performance of the transistor I and thereby adversely affects any circuit that utilizes the transistor 1. In a rectangular layout, the base pedestals 11 and the collector contacts 10 are capacitively coupled causing feedback between the collector and base areas. Furthermore, since the base-to-collector capacitance is proportional to the base pedestal areas, the large base pedestal area of conventional power transistors 1 increases the base to collector capacitance, thereby decreasing performance.
Accordingly, there remains a need for a transistor that occupies less area and is suitable for low operating voltages, while maintaining performance and reliability of the transistor.